1. Field of the Invention
This invention relates to gas-liquid contacting devices and the use of such devices in liquid treatment systems. The invention additionally relates to oxygen-absorption processes requiring repeated and prolonged air-liquid contact in sequential stages. The invention especially relates to methods and apparatus for aeration pumping of sewage in aerobic purification systems such as oxidation ditches and aeration ponds. The invention further relates to processes for treating slurries, particularly water treatment processes employing granular carbon.
2. Review of the Prior Art
Many processes have been developed for absorbing a gas in a liquid in order to effect a chemical reaction that precipitates a solute, decomposes a dissolved compound, bleaches suspended fibers, or forms a desired solution, for example. Some liquid treatment processes require that a gas be absorbed in the liquid in order to support living organisms such as fish or bacteria. Among the liquid treatment processes that support bacteria with dissolved oxygen, commonly termed aerobic processes, a large proportion treat aqueous wastes such as municipal sewage, cannery wastes, dairy wastes, meat-processing wastes, and the like.
Such aerobic processes are commonly accelerated by concentrating and activating the bio-organisms, termed bio-mass or activated sludge, and returning this sludge to be mixed with incoming wastewater which supplies food for the organisms. Activated-sludge processes for aerobic treatment of wastewaters have followed two main lines of development: vertical-flow aeration basins and circuit-flow oxidation ditches.
In an early oxidation-ditch process, Dutch Pat. No. 87,500 discloses horizontally mounted rotors having brush surfaces for adding oxygen to sewage and causing the sewage to flow for a period of time in a closed-loop circuit within an ovally laid-out ditch, the liquid then being clarified by settling and excess sludge being removed. In subsequent developments directed to adding oxygen to sewage and inducing circuit-flow circulation in oxidation ditches, U.S. Pat. No. 3,336,016 discloses an S-shaped duct, U.S. Pat. No. 3,510,110 the combination of a longitudinal partition and a vertically disposed surface aerator which is adjacent thereto, and U.S. Pat. No. 3,846,292 a plurality of subsurface ejector aerators.
Finally, U.S. Pat. No. 3,900,394 discloses a sewage purification process, to be carried out in a circuit-flow oxidation ditch having an impeller-type aerator at one or both ends, which comprises sequential aeration of incoming sewage, aerobic decomposition and depletion of its oxygen content, introduction of additional sewage to the oxygen-starved bacteria, and, simultaneously, aerobic decomposition and denitrification of the additional sewage as the bacteria break down its nitrates.
Returning to the concept of vertical-flow aeration basins, U.S. Pat. No. 1,247,540 teaches a spaced array of subsurface air diffusers in a tank. U.S. Pat. No. 3,452,966 discloses an open-ended vertical tube, with a helical baffle therein for creating turbulence and mixing air and liquid, which is submerged in sewage liquid with its lower end above a gas bubble generator. U.S. Pat. No. 3,479,017 also describes one of several impeller aerators which are vertically disposed for beating the surface of a basin while causing vertical circulation of its contents. U.S. Pat. No. 3,664,638 discloses a static mixer which is presently used as the preferred subsurface aeration device in sewage aeration basins, particularly when mounted as in FIG. 2 of the drawings and spaced in close array on the of the basin bottom.
Vertical-flow aeration basins are typically aerated on a large scale with one or more impeller-type aerators which are vertically mounted and disposed at the surface of the liquid, as discussed in Water & Wastes Engineering, Sept. 1975, pages 76-79, using an aerator such as is described in U.S. Pat. No. 3,479,017 and producing a uniform dissolved-oxygen (D.O.) content of 2.0 mg/liter. Such impeller aerators are frequently mounted within and at the upper open end of a draft tube extending partially or entirely to the bottom of the basin so that the aerator can more efficiently pump liquid from the bottom of the basin, having a depth up to 40 feet, and disperse it over the surface of the basin, thereby improving vertical circulation over a wide area. When fitted with a gear reducer to spin a nine-foot diameter impeller at low speeds, oxygen transfer efficiencies of 3.5 pounds O.sub.2 /hp/hour have been approached.
Returning to the circuit-flow oxidation ditch concept as presently put in practice, only horizontally mounted surface aerators, vertically mounted surface aerators without draft tube devices, and subsurface ejector aerators are used in oxidation ditches having closed-loop or circuit-flow circulation. No static mixers are employed in such oxidation ditches even though subsurface static aeration devices have oxygen transfer efficiencies, rated on a basis of pounds of oxygen transferred to or dissolved in liquid per brake horsepower per hour or per kilowatt-hour, that are equal to or better than the vertically mounted surface aerators and other aeration devices that are presently used in the sewage treatment field. A major advantage of aeration systems using the subsurface static aerator or mixer is that no moving mechanical parts or electrical motors are in contact with or near the liquid being treated because aeration is accomplished by concurrent passage of diffused air and liquid through and mixing within an open-ended vertical tube or conduit having a static internal mixing apparatus. Accordingly, a means for utilizing static mixers in oxidation ditches, while producing circuit-flow circulation therein, is greatly needed.
It is also unfortunate that both horizontally and vertically mounted surface aerators must be used at a relatively fixed level, although the impeller blades of a vertically mounted surface aerator can be slightly varied as to submergence in accordance with dissolved-oxygen content of the liquid being aerated. In contrast, static-mixer aerators can be used in aeration basins in which the surface is varied by two feet or more and also have no limitation as to depth of installation. Accordingly, a depth-variation means is needed for operating impeller aerators within operation ditches in which the depth is varied by several feet in order that these ditches can be available for additional use as storage facilities and for flow equalization of incoming wastewaters.
A basic consideration for this invention derives from the flow pattern, in both oxidation ditches and aeration basins, being principally vertical, with some accompanying horizontal flow in the former. Contact of liquid with air or oxygen is random. No method is available for controlling liquid circulation or frequency of liquid-gas contact. Although frequency of liquid-gas contact appears to be inconsequential because mixing of liquid having various contents of dissolved oxygen soon produces a uniform average content, it is quite important from an efficiency viewpoint. This is so because the necessary driving force increases non-linearly as the dissolved-oxygen content increases.
In consequence, if a portion of the liquid, initially having zero dissolved oxygen, contacts a gas such as air several times, it at first absorbs oxygen very readily but increasingly slowly thereafter. Vertical circulation causes some aerated water to be directly back-mixed into the intake of the aerator. Thus, energy is wasted by attempting to re-aerate water that has already been aerated. A need consequently exists for a flow control method and means for minimizing vertical circulation and turbulent mixing and for bringing liquid and gas into singly occurring contact.
Furthermore, accelerating a mass of liquid to a flow velocity of one foot per second (fps) and decelerating such flow to zero velocity, if in situ clarification in an oxidation ditch is desired, requires both energy and time. A need thus exists for a flow control means for rapidly accelerating and decelerating a mass of liquid, without the random mixing and vertical circulation that presently occurs, in order to combine clarification and aerobic digestion in a sequential procedure within the same oxidation ditch.
Static mixers are hereinafter termed static aerators, whether or not the gas being mixed is air. Impeller-type and submerged-turbine aerators are hereinafter termed surface aerators unless otherwise specifically defined. An aerator is in general a liquid-gas contact pump which is hereinafter to be understood to include a static aerator, a surface aerator, and a fountain discharging a liquid as a jet, spray, and the like into a gaseous atmosphere, such as air.
In prior art treatment of wastewaters, relatively dense foreign objects, which are commonly termed grit, are removed from raw sewage is separate treatment facilities before being sent to an oxidation ditch. A need thus exists, as a matter of simplicity and economy, for a grit-removal method and apparatus by means of which grit can be removed within an oxidation ditch without disturbing its normal aerobic digestive functions so that raw sewage influent can be sent directly from a collection facility to an oxidation ditch without an intermediate grit-removal step.
However, in prior art oxidation ditches, such as those sketched in FIGS. 1 and 4 of the drawings, translational movement of the liquid is created and roughly controlled merely by the momentum effect resulting from the motion of vertically disposed surface aerators, horizontally disposed surface aerators (brush type), or submerged ejector aerators. Turbulence, vertical currents, and non-uniform translational flows interfere with settling of grit in such an oxidation ditch. There is accordingly a concurrent need for a method and means for producing a highly uniform, accurately controlled, and plug-type flow of liquid within an oxidation ditch whereby grit settling and removal therewithin will be feasible.
Static aerators, as they are presently used in vertical-flow aeration basins, receive air from a dispersed grid of air-delivery lines, as indicated in FIG. 2 of the drawings, which are disposed along the bottom of a basin. If inspection, repair, or replacement of a portion of this grid must be performed, the entire basin may have to be partially or even entirely drained, and the static aerators may have to be disturbed. A need accordingly exists for an air delivery means which can be installed in an oxidation ditch and which can be separately removed, inspected, and repaired or replaced without disturbing the static aerators and without requiring drainage of the oxidation ditch.
Static aerators, as presently used for aerating wastewaters, are verticaly disposed and produce not only energy-wasteful mixing of non-aerated water with aeratored water but also vertical circulation of water without appreciable translational movement thereof. A need thus exists for an energy-conserving method and means for horizontally directing the flow of aerated water as it leaves the static aerator.